Tandem Isotope Therapy with 225Ac- and 177Lu-PSMA-617 in a Murine Model of Prostate Cancer

Visual Abstract

Radi onuclide therapy targeting the prostate-specific membrane antigen (PSMA) protein is a promising option for patients with metastatic castration-resistant prostate cancer.The most commonly used therapeutic isotope is 177 Lu, a medium-energy b-emitter (0.5 MeV) with a 6.7-d half-life (1).Response rates to 177 Lu-PSMA-617 (as measured by .50%decline in serum biomarker prostate-specific antigen) have varied from 30% to 70% across treatment studies and patient cohorts (2)(3)(4)(5)(6)(7).With a tissue penetration range of less than 2 mm and the ensuing cross-fire effect, 177 Lu b-particles are best suited for treating lesions of a certain size (i.e., nonmicroscopic) (8)(9)(10).Although treatment with 177 Lu-PSMA-617 is largely well tolerated, with a favorable dosimetry profile (11)(12)(13), in the setting of diffuse bone marrow infiltration, treatment with b-emitters may be limited by hematologic toxicity due to the irradiation of the surrounding healthy bone marrow tissues (10). 225Ac, an a-emitter with a 9.9-d half-life, has emerged as an alternative isotope with favorable therapeutic decay properties.a-particles are of interest in radionuclide therapy because of higher energy deposition over a much shorter tissue penetration range, causing more dense ionizations and localized dose profiles than for b-particles (14)(15)(16).The higher energy of 225 Ac a-particles over a shorter tissue range (,0.1 mm) leads to a linear energy transfer on the order of 100 keV/mm (9,(17)(18)(19).Decay schemes for 177 Lu and 225 Ac are shown in Figure 1.
Although fewer clinical studies have been conducted with 225 Ac-PSMA-617, studies so far have reported biochemical response rates (.50% decline in prostate-specific antigen) ranging from 25% to over 90%, though patient cohorts, prior treatments, and treatment settings have varied widely (20)(21)(22)(23).Additionally, a-particle therapy may be favored in the treatment of microscopic metastatic disease and bone marrow infiltration because of the shorter range of a-radiation (24)(25)(26)(27).However, the same radiobiologic features that make 225 Ac attractive against tumors also present a tradeoff at the expense of higher-grade toxicities (22).The most significant adverse effect of a-particle PSMA-targeted radionuclide therapy with small molecules (i.e., PSMA-617, PSMA-I&T) is xerostomia, making the salivary glands a key dose-limiting organ (28).Although the accumulation of PSMA ligands in the salivary glands is still not well understood, it is believed to be the result of both nonspecific (non-PSMA-related) and specific (PSMA-related) uptake mechanisms of PSMA ligands (29)(30)(31).Although xerostomia from 177 Lu therapy is often temporary and reversible, with 225 Ac there is a greater incidence of xerostomia, which can significantly diminish patient quality of life and lead to treatment discontinuation (22,32,33).
Overall, clinical experience using 177 Lu or 225 Ac has shown encouraging treatment responses; however, the responses have not been durable.Given that salivary gland toxicity limits the injected activity of 225 Ac, simply using higher 225 Ac treatment activities to increase tumor dose delivery is not an option.Dual-isotope combinations, or "tandem" approaches, may provide the benefits of both 177 Lu and 225 Ac to improve treatment tolerability while retaining high tumor dose delivery (10,34).Early clinical studies on both 177 Lunaïve patients and those who progressed after 177 Lu-PSMA have shown that augmentation of 177 Lu-PSMA therapy with a boost of 225 Ac is an effective option with a more favorable side effects profile (35)(36)(37).No incidents of grade 3 or higher xerostomia were reported in these studies.A similar approach has been tested in peptide receptor radionuclide therapy against neuroendocrine tumors using tandem 177 Lu/ 90 Y-DOTATATE, combining 2 b-particle-emitting isotopes with different energies and tissue penetration ranges (38,39).In the preclinical setting, tandem 177 Lu/ 90 Y-DOTATATE demonstrated an antitumor effect superior to that of either monotherapy alone (40).For PSMA-targeted therapies, clinical data are still limited by small patient cohort sizes, and to date, there have been no systematic preclinical investigations into the efficacy of dual-isotope combinations.In this work, we sought to directly compare aversus b-particle PSMA radionuclide therapy, as well as combinations of the two, in a mouse model of prostate cancer.Our hypothesis was that the emitted particle pathlength can impact the radiation dose delivery, especially to microscopic disease.The objective was to compare the treatment efficacy of a scaled combination of 225 Ac and 177 Lu to single-isotope treatments, as measured by longitudinal tumor control and survival.First, we conducted an ex vivo g-counting biodistribution and tumor dosimetry study to determine injected activities of 177 Lu and 225 Ac that yield comparable tumor doses.We then treated mice bearing disseminated prostate cancer lesions at 2 different stages of disease with 177 Lu-and 225 Ac-PSMA-617 as single agents, or in combination, to compare therapeutic efficacy and survival.

Cell Culture and Animal Studies
In all studies, the human-derived, PSMA-expressing prostate cancer tumor cell line C4-2 was used (courtesy of Dr. George Thalmann, Inselspital Bern, Switzerland).Cells were maintained in RPMI 1640 medium supplemented with 10% fetal bovine serum (Omega Scientific) and grown at 37 C and 5% CO 2 .Cells were monitored for Mycoplasma contamination using the Venor GeM Mycoplasma detection kit (Sigma Aldrich) and authenticated by short tandem repeat sequencing (Laragen).The parental cells were engineered to express firefly luciferase (C4-2-luc) to allow luciferase-mediated bioluminescence imaging to monitor tumor burden, as previously described (41).
All animal studies were approved by the UCLA Animal Research Committee (approval 2005-090).The mice were housed under pathogenfree conditions with food and water ad libitum and a 12 h-12 h lightdark cycle.Veterinary staff and investigators observed the mice daily to ensure animal welfare.
Radiochemistry PSMA-617 precursor was obtained from ABX Advanced Biochemical Compounds. 177Lu was obtained from Spectron MRC, and 225 Ac was supplied by the U.S. Department of Energy's Isotope Program within the Office of Science.Radiolabeling was performed as previously described with molar activities of 84 GBq/mmol and 130 MBq/mmol for 177 Luand 225 Ac-PSMA-617, respectively (41,42).
Biodistribution and Tumor Dosimetry of 177 Lu-and 225 Ac-PSMA-617 Immunodeficient, 6-to 8-wk-old NOD SCID g (NSG; The Jackson Laboratory) male mice were inoculated subcutaneously with 5 3 10 6 C4-2 cells in 100 mL of Matrigel (Corning) into the shoulder region (50 mice).After 3 wk, when the tumors reached about 300 mm 3 in volume, the mice were treated with either 30 MBq of 177 Lu-PSMA-617 (25 mice) or 40 kBq of 225 Ac-PSMA-617 (25 mice).The treatment activities were based on efficacious and well-tolerated activities in previous studies (43).The mice were euthanized at 1, 4, 24, 48, and 168 h after treatment (5 mice per time point for each nuclide).At the time of euthanasia, tumors and organs (including kidneys, liver, submandibular salivary glands, and intestines) were collected for ex vivo g-counting for activity quantification ( 177 Lu energy window, 189-231 keV; 225 Ac energy window, 170-260 keV for 221 Fr daughter detection; Cobra II Auto-Gamma; Packard Instrument Co.).Actinium samples were counted after 24 h when secular equilibrium was reached (44).The multiple t test method with Welch correction was used for biodistribution statistical comparisons (statistical significance set to #0.05).
We estimated tumor self-doses (ignoring cross-dose contributions from neighboring organs) by first curve-fitting and integrating the tumor time-activity curves (NUKFIT Software) (45).The total number of disintegrations was multiplied by dose constants to yield tumor doses for 177 Lu and 225 Ac (5.934 3 10 21 and 2.838 3 10 23 GyÁg/[mCiÁh], respectively).Dose constants are derived from nuclear data for energy released per disintegration of each radionuclide, ignoring contributions with a decay yield of less than 1% (1,46).In this case of self-dose calculation, it is assumed that all radiation has an absorption fraction of 1.0 and that all disintegrations measured in the tumor deposit all energy in the tumor.We could thereby estimate the injected activities of Lu-and 225 Ac-PSMA-617 that yield approximately equal tumor doses for subsequent studies directly comparing single-versus dualisotope approaches.
Tandem 177 Lu/ 225 Ac Therapy All subsequent therapy studies were investigated in a mouse model of advanced metastatic prostate cancer.NSG mice were inoculated with C4-2-luc cells in the left ventricle, leading to disseminated visceral and bone lesions, as previously described (80 mice) (41).The mice were treated at 2 different stages of disease-either 3 or 5 wk after inoculation with equivalent tumor dose-depositing activities of 177 Lu-or 225 Ac-PSMA-617 or in scaled combination.Three weeks after inoculation, the disseminated lesions are approximately 200 mm in size, increasing to millimeter scale by 5 wk, as previously characterized (41).The treatment activities were determined by the tumor biodistribution and dose-finding study as previously described.The treatment groups were as follows: 35 MBq of 177 Lu-PSMA-617, 40 kBq of 225 Ac-PSMA-617, a mixture of 17 MBq of 177 Lu-PSMA-617 plus 20 kBq of 225 Ac-PSMA-617, or untreated (10 mice per group per treatment time, intravenous administration; the chosen activities are justified in the Results section).The tandem-isotope treatment regimen was designed to halve the respective doses of 225 Ac and 177 Lu in combination.
Disease burden was assessed by weekly luciferase-mediated bioluminescence imaging (IVIS Lumina III; Perkin Elmer), and the mice were followed for overall survival.The mice were euthanized when their overall condition showed signs of deteriorating health based on the body conditioning score (47).Therapeutic efficacy data regarding tumor burden, as measured by whole-body radiance over time (Living Image; Perkin Elmer), were analyzed with 1-way ANOVA with Bonferroni adjustment using GraphPad Prism 8.The log-rank (Mantel-Cox) test was used for survival analysis.

RESULTS
Biodistribution and Tumor Dosimetry of 177 Lu-and 225 Ac-PSMA-617 The ex vivo biodistribution of 177 Lu-and 225 Ac-PSMA-617 in subcutaneous tumors and organs is shown in Figure 2. Tabulated biodistribution values for tumors and organs are available in Supplemental Tables 1 and  2 (supplemental materials are available at http://jnm.snmjournals.org).Kidney uptake at 1 h after injection was significantly greater for 225 Ac than for 177 Lu (25.9% 6 3.5% and 13.3% 6 1.6% injected activity/g, P , 0.0005), but no statistical difference was observed at later time points.Both 177 Lu-and 225 Ac-PSMA-617 rapidly localized to the tumor, with similar uptake (14.4% 6 4.1% and 14.1% 6 4.9% injected activity/g, respectively) at 4 h after intravenous injection (not statistically significant; P 5 0.89).Tumor uptake peaked at 4 h after injection, and by 168 h (7 d), tumor uptake was significantly greater in 225 Ac-treated tumors than in 177 Lu-treated tumors (9.3% 6 1.3% vs. 6.3% 6 0.9% injected activity/g, P , 0.004, 5 mice per radionuclide).Tumor time-activity uptake curves in percentage injected activity were used for curve-fitting in NUKFIT (Figs. 2B and 2D).NUKFIT software selected the best curve-fitting model in both cases to be in the following form: A 1 e 2l1t 1A 2 e 2l2t , where l 1 and l 2 represent the fitted decay constants.The best-fit curve parameters are shown in Figure 2. The resultant cumulated activities were multiplied by the respective dose constants and normalized by injected activity and average tumor masses.The tumor-absorbed doses for 177 Lu-and 225 Ac-PSMA-617 in the subcutaneous C4-2 model were 0.00758 and 65.03 cGy/kBq, respectively, or roughly 850 times greater for 225 Ac than for 177 Lu.Therefore, from this study we decided to use 35 MBq of 177 Lu and 40 kBq of 225 Ac to yield similar absorbed tumor doses in the therapeutic efficacy studies.Previous work demonstrated that 40 kBq of 225 Ac-PSMA-617 is well tolerated (43).For the tandem treatment arm, we chose to halve the activity of each isotope in combination (17 MBq of 177 Lu 1 20 kBq of 225 Ac) to test a more tolerable activity regimen.
Tandem 177 Lu/ 225 Ac Therapy Whole-body bioluminescence imaging radiance over time and mouse survival curves for treatment 3 wk after inoculation are shown in Figures 3 and 4, respectively.Notably, the disease burden of mice treated at this earlier stage of disease with 177 Lu-PSMA-617 was not significantly different from that of untreated mice at any time point (P 5 0.932).However, 225 Ac-PSMA-617 both as a single agent and in combination with 177 Lu-PSMA-617 was associated with significant tumor growth retardation (P 5 0.009 for tandem vs. 177 Lu; P 5 0.0084 for225 Ac vs. 177 Lu; P .0.999 for tandem vs. 225 Ac measured 5 wk after treatment).If mice were left untreated, median survival was 8.3 wk.When mice were treated with 35 MBq of 177 Lu-PSMA-617, no significant survival benefit was observed (median survival, 9.4 wk; P 5 0.337).However, when treated with one of the 225 Ac regimens, the median survival increased to 14.1 wk for the tandem approach and 15.3 wk for 225 Ac-PSMA-617 alone (P , 0.0001 for tandem vs. 177 Lu; P , 0.0001 for 225 Ac vs. 177 Lu; P 5 0.108 for 225 Ac vs. tandem).When mice were treated at a later time point of macroscopic disease (5 wk after inoculation), all treatment groups showed retarded tumor growth relative to untreated mice (Fig. 5).However, the greatest benefits were observed with 225 Ac-PSMA-617 monotherapy and tandem approaches (P , 0.0001 for 225 Ac vs. 177 Lu; P , 0.0001 for tandem vs. 177 Lu measured 5 wk after treatment).Median overall survival increased from 7.9 wk (untreated) to 10.3 wk for 177 Lu-PSMA-617, 13.2 wk for tandem therapy, and 14.6 wk for 225 Ac-PSMA-617 alone (P , 0.0001 for NT vs. 177 Lu; P , 0.0001 for tandem vs. 177 Lu; P , 0.0001 for 225 Ac vs. 177 Lu) (Fig. 6).There were no significant differences in whole-body disease burden or survival benefit conferred between 225 Ac alone and halving the 225 Ac activity in tandem with 177 Lu (P 5 0.171 for 225 Ac vs. tandem survival and P .0.999 for 225 Ac vs. tandem whole-body radiance 5 wk after treatment).

DISCUSSION
To our knowledge, this was the first study reporting on the efficacy of 177 Lu/ 225 Ac-PSMA tandem-isotope combinations in a mouse model of prostate cancer.In this work, we sought to compare the treatment efficacy of the same tumor dose delivered by 3 different radiation mechanisms: b-particles ( 177 Lu), a-particles ( 225 Ac), or both ( 177 Lu 1 225 Ac).To do so, we first determined comparable injected activities to yield comparable tumor doses by conducting a full ex vivo biodistribution study using both 177 Lu-PSMA-617 and 225 Ac-PSMA-617.It is important to note that the applied injected activities were chosen so as to expose the cancerous lesions to comparable absorbed radiation doses, without adjustment for the effectiveness of the type of radiation.Interestingly, there was no significant difference in peak tumor uptake of 177 Lu-or 225 Ac-PSMA-617 at 4 h after injection; however, increased tumor retention was observed at all subsequent time points for mice treated with 225 Ac-PSMA-617 compared with 177 Lu-PSMA-617.Although normal-organ biodistribution was not the primary objective of this report, all biodistribution data are available in the supplemental materials.In addition to the kidneys, liver, salivary glands, and intestines included in this report, uptake data for the following additional organs are available: blood, heart, lungs, spleen, stomach (with contents), prostate, testes, muscle, femur (with and without bone marrow), bone marrow, and brain.
In this work, we challenged a tandem-isotope approach against an advanced model of widespread disease to compare the dynamics of tumor control and overall survival.To more objectively compare the treatment arms against one another, we first determined injected activities for 177 Lu and 225 Ac that expose the tumors to similar absorbed doses in the subcutaneous C4-2 model.We then designed the tandem arm to be a combination of half of each respective single-agent activity.The tandem-isotope approach was tested with the intracardiac inoculation model of C4-2 cells, at 2 different stages of disease progression with different volumes of lesions.Although subcutaneous tumors allow for straightforward uptake quantification, and therefore interrogation into absorbed doses, they fail to recapitulate the metastatic state.Dosimetry was not feasible in the intracardiac inoculation model because the lesions are not easily isolated, especially in microscopic stages.Translating dosimetry findings from subcutaneous models is one approach toward standardizing the applied activity when the tumor burden is distributed throughout the mouse and direct dosimetry is not possible.Furthermore, we sought to investigate the performance of tandem therapy in a model that more faithfully represents the setting in which treatment with a-particles (alone or in combination) would actually be clinically warranted.
The intracardiac inoculation model is an aggressive prostate cancer model in which mice, when left untreated, succumb to   225 Ac-PSMA-617 alone (P 5 0.337 for NT vs. 177 Lu, P , 0.0001 for tandem vs. 177 Lu; P , 0.0001 for225 Ac vs. 177 Lu; P 5 0.108 for extensive disseminated disease warranting euthanasia by 8 wk after inoculation (Figs. 4 and 6).This model has previously been shown to be sensitive to treatment with 225 Ac-PSMA-617 early in the disease, but this work is the first effort, to our knowledge, at challenging at a very advanced stage (5 wk after inoculation) (41).By treating at 2 different stages of disease, we can also seek to answer what size lesions most benefit from treatment with 177 Lu or tandem approaches.At the onset of the earlier treatment time investigated (3 wk after inoculation), the disseminated lesions are approximately 200 mm in size, increasing to millimeter scale by 5 wk, as previously characterized (41).When given at 3 wk, 177 Lu-PSMA-617 as a single agent did not significantly improve tumor control or survival relative to untreated mice (Fig. 4).However, when lesions were millimeters in size at the time of treatment, 177 Lu-PSMA-617 retarded tumor growth and the mice conferred a significant survival advantage.Failure of 177 Lu-PSMA-617 against micron-scale lesions in the earlier treatment setting can be explained by a mismatch between the target lesion size and the pathlength of the therapeutic radiation.With a mean tissue range of 600 mm (9), b-particles from 177 Lu travel a distance that exceeds the lesion size at the time of early treatment (3 wk).This results in a loss of specificity to the targeted lesions and attenuates tumor response, as confirmed in this study.
Given the tissue range of less than 0.1 mm, a-particles from 225 Ac yield dense ionizing paths with little to no cross-fire effect.For treatment at both stages of disease, mice treated with 225 Ac-PSMA-617 as a single agent survived the longest and experienced the best tumor control (despite not reaching statistically significant differences at all time points).Even when challenged with lesions on the millimeter scale, the single agent 225 Ac-PSMA-617 outperformed 177 Lu-PSMA-617.Interestingly, halving the 225 Ac activity in tandem with 177 Lu did not significantly increase the whole-body disease burden as measured 5 wk after treatment (Figs. 3A and 5A).In this model, the comparable tumor control and survival between the full 225 Ac dose and the tandem dose regimen suggest that a reduced administered activity of Lu-and 225 Ac-PSMA-617 in the clinical setting, the applied injected activities have been heterogeneous.The mean reported activity of 177 Lu-PSMA-617 has ranged from 6 to 6.7 GBq, whereas the mean activity of 225 Ac-PSMA-617 has ranged from 2.7 to 4 MBq (35,37).Another study reported a median 177 Lu-PSMA-617 activity of 6.9 GBq (ranging from 5.0 to 11.6 GBq) and 5.3 MBq (ranging from 1.5 to 7.9 MBq) for 225 Ac-PSMA-617 (36).This wide range in applied activities precludes interpretation of a recommended activity scheme, and the heterogeneity underscores that, in practice, the treatment activities should reflect the individual patient condition.Additional studies are thereby needed to establish maximum tolerated doses in tandem-isotope schemes.One such prospective study examining the combination of 177 Lu-PSMA-I&T and 223 Ra (AlphaBet trial NCT05383079) is under way using a fixed 7.4 GBq of 177 Lu-PSMA administration followed by escalating activities of 223 Ra (48).Furthermore, prior treatments, including the cumulative injected activity of 177 Lu-PSMA-617 as a monotherapy if applicable, should also be considered in the activity prescription.Yet another factor to consider in the design of tandem-isotope treatments is the timing of the relative administrations.Although our preclinical work investigated the simultaneous administration of 177 Lu and 225 Ac, tandem treatment on consecutive days or in the first few days of each cycle also warrants investigation.Further work is required to understand the optimal conditions under which to prescribe tandem-isotope approaches, including how to adapt the treatment activities to reflect individual patient tumor burden, metastatic extent, and prior treatments.
One limitation of the mouse models used in these studies is the inability to recapitulate critical organ uptake as seen in patients (i.e., in the salivary glands and kidneys).In the subcutaneous tumor model, kidney uptake was higher with 225 Ac than with 177 Lu  at 1 h after injection; however, no significant difference was observed beyond the initial uptake phase.Clinically, kidney and salivary gland equivalent doses from 177 Lu-PSMA-617 were reported in one study as 0.39 and 0.36-0.58Sv/GBq, respectively (13).For 225 Ac-PSMA-617, one dosimetry report calculated kidney and salivary gland doses to be 0.74 and 2.33 Sv/MBq, respectively (assuming a relative biologic effectiveness factor of 5 for 225 Ac) (49).However, our biodistribution study revealed no significant differences in submandibular gland uptake between 177 Lu-and 225 Ac-treated mice, and peak uptake was less than 0.5% of injected activity at all measured time points (Fig. 2).Given this inherent limitation in the translatability of salivary gland toxicity, it was not within the scope of this study to assess the preclinical feasibility of tandem-isotope treatment to improve the toxicity profile.

CONCLUSION
Treatment of a microscopic model of prostate cancer with 40 kBq of 225 Ac-PSMA-617 or 20 kBq of 225 Ac in tandem with 17 MBq of 177 Lu resulted in significantly decreased tumor growth compared with 177 Lu, which was ineffective as a single agent against microscopic lesions, likely because of a mismatch of particle pathlength and lesion size.Mice treated later (when lesions were millimeter scale in size) experienced significant tumor growth retardation and survival benefit in both monotherapy and tandem regimes of 177 Lu-and 225 Ac-PSMA therapy.However, the greatest benefits were observed with 225 Ac-PSMA-617 as a single agent and in tandem approaches.Further work is needed to identify the disease patterns and settings that most benefit from treatment with b-particles, a-particles, or both.

DISCLOSURE
Katharina Lueckerath reports paid consulting activities for Sofie Biosciences/iTheranostics and funding from AMGEN outside the submitted work.Jeremie Calais reports prior consulting activities for Advanced Accelerator Applications, Blue Earth Diagnostics, Curium Pharma, GE Healthcare, EXINI, IBA RadioPharma, Janssen Pharmaceuticals, Lantheus, POINT Biopharma, Progenics, Radiomedix, and Telix Pharmaceuticals.No other potential conflict of interest relevant to this article was reported.

KEY POINTS
QUESTION: Does dual-isotope tandem 177 Lu/ 225 Ac-PSMA radionuclide therapy improve treatment outcomes in a mouse model of disseminated prostate cancer?
PERTINENT FINDINGS: We determined equivalent tumor dose-depositing injected activities of 177 Lu-and 225 Ac-PSMA-617 and tested treatment as single agents versus in combination against different-sized lesions. 225Ac alone, and in tandem with 177 Lu, was superior to 177 Lu as a single agent against microscopic lesions.No significant difference was observed between the tandem and 225 Ac treatment arms.

IMPLICATIONS FOR PATIENT CARE:
This work provides efficacy data for 177 Lu-and 225 Ac-PSMA-617 as single agents and in combination against lesions of micron to millimeter scale.These data can be used as a basis for further investigations into the disease conditions best suited for treatment with b-particles, a-particles, or both.

FIGURE 2 .
FIGURE 2. Ex vivo biodistribution and tumor time-activity curves for 177 Lu-and 225 Ac-PSMA-617.(A) Mean 6 SD percentage injected activity per gram of tissue for mice treated with 30 MBq of 177 Lu-PSMA-617 (5 mice per time point).Tabulated values and additional organs are available in supplemental materials.(B) Mean percentage injected activity of 177 Lu-PSMA-617 in tumors over time used for curve-fitting and dosimetry.Best-fit parameters are shown.(C) Mean 6 SD percentage injected activity per gram of tissue for mice treated with 40 kBq of 225 Ac-PSMA-617 (5 mice per time point).Tabulated values and additional organs are available in supplemental materials.(D) Mean percentage injected activity of 225 Ac-PSMA-617 in tumors over time used for curve-fitting and dosimetry.Best-fit parameters are shown.

FIGURE 3 .
FIGURE 3. Therapeutic efficacy for mice treated 3 wk after inoculation with 177 Lu/ 225 Ac-PSMA-617 or in tandem.(A) Mean 6 SD whole-body radiance over time (10 mice per group).Data are shown for time points with 5 or more remaining mice.(B) Individual mouse fold change in radiance over time relative to disease burden at time of treatment.NT 5 no treatment.

FIGURE 5 .
FIGURE 5. Therapeutic efficacy for mice treated 5 wk after inoculation with 177 Lu/ 225 Ac-PSMA-617 or in tandem.(A) Mean 6 SD whole-body radiance over time (10 mice per group).Data are shown for time points with 5 or more remaining mice.(B) Individual mouse fold change in radiance over time relative to disease burden at time of treatment.NT 5 no treatment.